1. Fields of the Invention
This invention is directed to an automated blood pressure monitoring instrument, more particularly to a non-invasive detection instrument for automatically measuring arterial blood pressures by an indirect auscultation technique.
2. Description of the Prior Art
There have been provided a wide variety of blood pressure measuring instruments based on listening for the Korotkoff sounds. The common technique with the use of such instrument is to place an inflatable cuff around the arm of a user and to occlude the artery in the arm by increasing the pressure within the cuff above an expected systolic pressure by about 20 to 30 mmHg. Thereafter, the pressure in the cuff is allowed to bleed down slowly at a ratio of about 2 to 3 mmHg/sec for detecting the Korotkoff sounds by means of a pressure transducer such as a microphone or the like. The pressures in the cuff at the time of the appearance and disappearance is then recognized by the instrument to be the indications of the systolic and diastolic pressures. However, such prior art instruments have long suffered problems that the detection by the instruments is not always in exact coincidence with the detection by doctors or the like personnel who have been acquainted with the blood pressure measurement using a conventional stethoscope together with a mercury manometer and that detection based upon the human ear has long been widely established and recognized as the standard reference for indicating the systolic and diastolic pressures. There is an inability for the artificial sound sensing means to automatically adjust a critical level, while such automatic critical level adjusting ability is inherent to the human ear and is most reliable for recognizing incoming sounds of important intensity by relative comparison thereof with the self-adjusting critical level in such a way as not to be substantially affected by possible noises. In other words, the human ear has the filtering function for a target sound in such a manner as to ignore possible noises, thus recognizing the target sound as having a larger level difference between the target and noises than actually exists. This lack in self adjusting ability of the critical level may also be the cause of mistaking the noises for the Korotkoff sounds when the noise is larger and therefore may result in erroneous blood pressure measurements. In fact, there has never been provided a blood pressure monitoring instrument capable of automatically adjusting its critical level depending upon a level of noises inevitably emanated from the occluded artery and/or the body during the measurement. Accordingly, it is most desirable to present an automated blood pressure monitoring instrument capable of ignoring the noises so as to provide an accurate measurement approximating the measurement by skilled personnel.
In addition, the above inconsistency between the blood pressure measurement by the prior art automated instrument and by the human ear can be reasonable explained also in terms of the fact that the skilled personnel rely on a conventional stethoscope to listen for the Korotkoff sounds while the automated instrument receives the Korotkoff sounds through the sound sensing means such as the microphone placed rather directly on a suitable portion of the body. Therefore, this structural difference with respect to the sound conducting path should be taken into account for approximating the measurement by the automated instrument to that by the auscultation technique with the conventional stethoscope. In connection with the above, a particular attention should be directed to the difference in frequency response between the artificial sound sensing means employed and the human ear for obtaining the measurement results as close as those by the skilled personnel.
In the meanwhile, the Korotkoff sounds upon which both the automated instrument and the human ear rely for determining the systolic and diastolic blood pressures have been analyzed by Swan to exhibit a unique spectrum during the blood pressure measurement. That is, the spectrum of the Korotkoff sounds is found to have five remarkable points S.sub.1 to S.sub.5 and four corresponding phases I, II, II, IV between the adjacent points, S.sub.1 being defined to be indicative of the systolic pressure and S.sub.5 or S.sub.4 indicative of the diastolic pressure. Such unique spectrum of the Korotkoff sounds has been practically found to successively appear in correct or proper measuring procedure, thus constituting a standard for determining whether or not the measurement is being properly performed. Particularly worthy of mention among the characteristics of this spectrum is the appearance of a voiced tone associated with a particular Korotkoff sound, or the Korotkoff sound in the second phase S.sub.2. This can be an easy but strong standard for the judgement of the effectiveness of the measurement. The voiced tone is found to appear immediately after the peak of the Korotkoff sound in the second phase S.sub.2 if the measurement is being properly performed. In fact, the condition in which said voiced tone appears takes the advantage of producing the Korotkoff sounds of a greater sound pressure than at the condition in which the voiced tone fails to appear, thus giving an increased S/N ratio for easy and accurate measurement. It is of course within the recognition of the above skilled personnel to listen for this voiced tone for determining their measuring procedure to be effective or not. Accordingly, it is strongly desired for the automated measuring instrument to discriminate the voiced tone associated with the particular Korotkoff sound in view of determining the effectiveness of its measurement.